Process costing is suitable for .a.brick laying firms.b.oil reefing firms.c.hospitals.d.transport firms.

Process costing is suitable for:

a. Brick laying firms: This is not typically suitable for process costing. Brick laying firms usually use job costing because each project is unique and costs can vary greatly from one job to another.

b. Oil refining firms: This is suitable for process costing. Oil refining firms typically produce large quantities of a homogeneous product, making process costing an appropriate method to calculate costs.

c. Hospitals: This is not typically suitable for process costing. Hospitals usually use job costing because each patient's treatment is unique and costs can vary greatly from one patient to another.

d. Transport firms: This is not typically suitable for process costing. Transport firms usually use job costing because each transport job is unique and costs can vary greatly from one job to another.

So, the most suitable for process costing among these options is oil refining firms.